Sensors according to the state of the art often are provided as integrated sensors on semiconductor chips.
Such sensors may be humidity sensors which use a layer of a humidity sensitive material arranged on a semiconductor chip, as it is described in WO 01/42776. Other types of sensors e.g. use metal oxide technologies and are adapted to measure various types of substances in gases or liquids. Typical substances that can be measured are e.g. CO, CO2, NOx, volatile organic compounds (VOC), in particular any type of gaseous organic compounds, and any other types of compound.
Semiconductor chips are usually manufactured in wafers, where each wafer may comprise hundreds or more chips. After manufacturing, the wafers are cut to separate the chips from each other, place them in a suitable housing and calibrate each sensor individually by exposure to fluids of known composition, as it is e.g. described in WO 01/40784.
For improving the calibration process it is suggested to calibrate the sensors prior to separating the semiconductor chips which also is called as calibration on a “wafer level”. For implementing such calibration routine, U.S. Pat. No. 7,281,405 B1 discloses an apparatus with a chuck for holding a wafer with sensors to be calibrated, and a lid arranged above the chuck and facing the wafer. The lid provides an opening in which a probe head is arranged. The probe head has the form of a flat plate fixed to the lid and an opening at its centre. Needle electrodes in form of cantilevers are fixed at the lower surface of the plate. The chuck may be moved laterally with respect to the lid such that the needle electrodes can be positioned on suitable pads on the wafer for producing calibration measurements. In order to expose the sensors on the wafer to a calibration environment, the lid includes a circular feed duct and small openings facing the wafer for providing calibration gas supplied via the feed duct and emanating from the small openings.
However, since the probe head is located in the middle of the lid and is fixed within the opening of the lid, there cannot be calibration gas supplied directly to this area. On the other hand, this area represents the location where the measurement is performed by means of the needle electrodes, such that it is unfortunate that this location cannot be directly exposed to the calibration gas.